EP3084450B1 - Optical sensor - Google Patents

Optical sensor Download PDF

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Publication number
EP3084450B1
EP3084450B1 EP13811242.0A EP13811242A EP3084450B1 EP 3084450 B1 EP3084450 B1 EP 3084450B1 EP 13811242 A EP13811242 A EP 13811242A EP 3084450 B1 EP3084450 B1 EP 3084450B1
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EP
European Patent Office
Prior art keywords
optical
fiber
phase shift
sensing element
sensor
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EP13811242.0A
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German (de)
English (en)
French (fr)
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EP3084450A1 (en
Inventor
Klaus Bohnert
Andreas Frank
Georg Müller
Lin Yang
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ABB Schweiz AG
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ABB Schweiz AG
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
    • G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
    • G01R15/24—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices
    • G01R15/245—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
    • G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
    • G01R15/24—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices
    • G01R15/247—Details of the circuitry or construction of devices covered by G01R15/241 - G01R15/246
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
    • G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
    • G01R15/24—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices
    • G01R15/245—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect
    • G01R15/246—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect based on the Faraday, i.e. linear magneto-optic, effect
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/032—Measuring direction or magnitude of magnetic fields or magnetic flux using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect
    • G01R33/0322—Measuring direction or magnitude of magnetic fields or magnetic flux using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect using the Faraday or Voigt effect

Definitions

  • the disclosure relates to an optical sensor with a sensing element causing a phase shift of light waves passing through it in the presence of a measurand field, such as a fiber optic current sensor (FOCS) or magnetic field sensor that includes a sensing fiber to be exposed to a magnetic field, e.g. of a current to be measured, as typically used in high voltage or high current applications.
  • a measurand field such as a fiber optic current sensor (FOCS) or magnetic field sensor that includes a sensing fiber to be exposed to a magnetic field, e.g. of a current to be measured, as typically used in high voltage or high current applications.
  • FOCS fiber optic current sensor
  • magnetic field sensor that includes a sensing fiber to be exposed to a magnetic field, e.g. of a current to be measured, as typically used in high voltage or high current applications.
  • Fiber-optic current sensors rely on the magneto-optic Faraday effect in an optical fiber that is coiled around the current conductor.
  • the current-induced magnetic field generates circular birefringence in the optical fiber that is proportional to the applied magnetic field.
  • a preferred arrangement employs a reflector at the sensing fiber's far end so that the light coupled into the fiber performs a round trip in the fiber coil.
  • left and right circularly polarized light waves are injected into the sensing fiber which are generated from two orthogonal linearly polarized light waves by a fiber optic phase retarder spliced to the sensing fiber and acting as quarter-wave retarder (QWR), as described in reference [1].
  • QWR quarter-wave retarder
  • the Verdet constant is material-, temperature-, and wavelength-dependent.
  • the senor may be designed as a Sagnac-type interferometer with quarter-wave retarders (QWRs) at both sensing fiber ends and light waves of the same sense of circular polarization that are counterpropagating in the sensing fiber (see ref. [1]).
  • QWRs quarter-wave retarders
  • High performance current sensors often use an interferometric technique based on non-reciprocal phase modulation as also applied in fiber gyroscopes in order to measure the optical phase shift, see e.g. ref. [2].
  • Integrated-optic phase modulators or piezo-electric modulators are employed.
  • the technique provides, particularly in combination with closed-loop detection, high accuracy, good scale factor stability, and a linear response over a large range of magneto-optic phase shift.
  • the technique is relatively sophisticated and often requires polarization-maintaining (PM) fiber components and elaborate signal processing.
  • PM polarization-maintaining
  • integrated-optic modulators are relatively expensive components.
  • the sensor accuracy may be sufficient for protection functions in high voltage substations (IEC accuracy class 5P demands an accuracy to within ⁇ 1% at the rated current), the accuracy typically is insufficient for electricity metering; the IEC metering class 0.2 for example demands an accuracy to within 0.2% at the rated current (reference [4]).
  • US 5895912 discloses a polarimetric AC sensor in which DC signal components are used to intensity-normalize the AC measurement signals.
  • optical sensors of the above kind such as magnetic field sensors or fiber-optic current sensors (FOCSs), and related methods that increase the accuracy of such sensors even when using passive optical components instead of actively phase modulating components to detect a relative phase shift between light waves.
  • FOCSs fiber-optic current sensors
  • the method is based on generating two sets of light waves having different velocities in the presence of a non-vanishing measurand field within a sensing element of the sensor and with a defined static bias phase shift introduced between the two sets of light waves, with the sensor converting a total optical phase shift including the static bias optical phase shift and the optical phase shift induced by said measurand field into optical power changes of opposite signs (anti-phase) in at least two detector channels, with the method including the step of normalizing the optical power changes after their conversion into electrical detector signals in two detector channels to reduce the effects of uneven intensity and different loss or gain in the detector channels.
  • the two sets of light waves are typically orthogonal linearly polarized light waves or left and right circularly polarized light waves.
  • the phase shift between both sets is converted into changes of optical power having opposite signs (i.e. being in anti-phase) in the two detector channels.
  • the static bias optical phase shift is typically around (2n + 1) x 90°, particularly within (2n + 1) x 90° ⁇ 20° or (2n + 1) x 90 ⁇ 5°, wherein n is any integer number.
  • the static bias optical phase shift can be introduced, e.g., by using at least one quarter-wave retarder or a Faraday rotator.
  • the sensing element can be an optical fiber element, a bulk magneto-optic material (such as a Yttrium iron garnet crystal or fused silica glass), or an optical fiber or bulk optic material attached to a magnetostrictive element.
  • the sensing element can be an electro-optic crystal [21], a crystalline electro-optic fiber [19], a poled fiber [20], or a fiber attached to a piezo-electric material [7].
  • the normalization of the optical power changes is derived from filtering spectral components from the electrical detector signals of the detection channels and combining the spectral components or at least one normalization factor derived therefrom with at least one detector signal to yield normalized detector signals.
  • the spectral components can be filtered AC components and/or transient components of at least one of the detector signals and/or (low-pass) filtered components or DC components of at least one of the detector signals.
  • the method in accordance with the present invention can be used for measuring DC, AC, or transient measurand fields.
  • the spectral components are time-averaged for noise reduction.
  • the AC spectral components are preferably in a range around a nominal frequency of the measurand field, such as in the range of 45 Hz to 65 Hz for the standard electric power grid frequencies. If amplitudes of AC and transient spectral components fall below a threshold, they can be replaced by either default values or by low-pass filtered signal components.
  • the combined sensor signal after normalization is preferable high-pass filtered.
  • a temperature of the (passive) optical elements introducing the static bias optical phase shift is derived from the low frequency or DC components of the sensor signal.
  • the DC or low frequency components depend on the static bias optical phase shift.
  • the bias phase shift can change as the temperature of the optical components introducing the phase shift changes and thus can be indicative of the component's temperature.
  • the DC or low frequency component of the signal can serve as a measure of the temperature at the location where the static bias optical phase shift is introduced, such as a suitable integrated-optic polarization splitter module, which can be used for this purpose.
  • This temperature compensation can be applied in combination with other temperature compensations, particularly with a temperature compensation of the sensing element, itself.
  • the senor includes at least: a light source and at least one light detector, preferably at least two light detectors; and at least two, preferably at least three, optical transmission channels, with one channel providing a forward channel for the light to a sensing element and one or two channels providing return detector channels for the light to the detectors; and one or more passive optical elements for introducing a static bias optical phase shift between two different sets of light waves, that have different velocities within said sensing element in the presence of a non-vanishing measurand field, and for converting a total optical phase shift including the static bias optical phase shift and an optical phase shift induced by the measurand field into optical power changes of opposite signs (anti-phase) in at least two detector channels; and a polarization maintaining (PM) fiber with the PM fiber being connected directly or indirectly via at least one retarder or a Faraday rotator element to the sensing element; with at least parts of the one or more passive optical elements being in thermal
  • At least parts of the one or more passive optical elements are at ground potential and the PM fiber provides an optical connection from ground potential to the potential of the sensing element, with the latter potential being different from ground potential and typically being a medium voltage or high voltage.
  • the PM fiber provides an optical connection from ground potential to the potential of the sensing element, with the latter potential being different from ground potential and typically being a medium voltage or high voltage.
  • the one or more passive optical elements for introducing a static bias optical phase shift and the one or more passive optical elements converting a total optical phase shift are best combined in an integrated optical polarization splitter module with at least two or three ports on the optical source/detector side and one port on the sensing element side, with the port on the sensing element side being connected to the PM fiber.
  • the senor can be adapted to measure AC or DC fields, in particular magnetic fields, current, electric fields, voltage, or force fields.
  • FIG.1 there is shown the example of a configuration of an optical current sensor using a static bias optical phase shift as signal detection scheme.
  • FIG. 1 For further details and possible modifications of the known elements of FIG. 1 reference can be made to [3].
  • the senor 10 includes an opto-electronics module 11 which houses for example a broad-band light source 111, e.g. a superlumines-cent diode, two photo detectors 12-1, 12-2, and a signal processing unit 113 with power supply and other electronic components (not shown).
  • a broad-band light source 111 e.g. a superlumines-cent diode
  • two photo detectors 12-1, 12-2 e.g. a superlumines-cent diode
  • a signal processing unit 113 with power supply and other electronic components (not shown).
  • any connection between the opto-electronics module 11 and a sensor head 13 with the sensing element 131 is purely optical and is made in the example shown by single mode fibers 101 and single mode fiber connectors 102.
  • the elements in the optical path outside the opto-electronics module 11 are passive optical components and hence do not require electrical activation.
  • the light from the light source 111 is depolarized, for example in a fiber Lyot depolarizer (not shown).
  • the (optical) connection between the opto-electronic module 11 and the sensor head 13 is made through three single-mode optical fibers 101 (SMF1, SMF2, SMF3), which connect to three source side ports of an integrated optical polarization splitter module 14, which in the example shown is based on an integrated-optic 1x3 splitter/combiner 140 (SC) forming the module together with the polarizers and retarders (141, 144, 145) attached to it.
  • SC integrated-optic 1x3 splitter/combiner 140
  • a first of such polarizers is polarizer 141 (P1), which polarizes the light at entrance to the SC 140 at a first port, also referred to as the SC light source channel, on the source side face.
  • the polarization direction is at 45° with the respect to the normal of the SC plane.
  • the waveguides within the SC 140 are of low birefringence in order not to alter the polarization state of the transmitted light.
  • the light is coupled into a polarization-maintaining (PM) fiber pigtail 132.
  • the principal axes of the PM fiber 132 are parallel and perpendicular to the SC plane, i.e. at 45° to the polarization direction of the polarizer P1 141.
  • both orthogonal polarization-modes of the PM fiber 132 are excited with the same amplitude.
  • a fiber-optic quarter-wave retarder 133 converts the orthogonal linearly polarized light waves into left and right circularly polarized waves before the light enters the sensing fiber 131.
  • the sensing fiber forms a coil with an integer number of fiber loops around a current conductor 15.
  • the light is reflected at the far end of the fiber 131 by a reflector 135 and then passes the coil a second time.
  • the polarization states of the two light waves are swapped, i.e. left circular light becomes right circular and vice versa.
  • the retarder 133 converts the reflected circular waves back to orthogonal linear waves.
  • the polarization directions of the returning linear waves are also swapped compared to the forward propagating waves.
  • the returning orthogonal waves have a magneto-optic phase shift ⁇ as a result of the Faraday effect (see also eq. [3] below).
  • the SC 140 splits the orthogonal light waves into two optical detection channels 142, 143.
  • a quarter-wave retarder plate (QWR) 144 at the source side face is used as the element to introduce a static bias optical phase shift, which in this example is a 90° differential phase delay between the orthogonal waves of both detection channels 142, 143.
  • the principal axes of the QWR 144 are aligned parallel to the axes of the PM fiber pigtail 132 and at 45° to polarizer P1 141.
  • the orthogonal waves of a first of the detector channels 142, 143 interfere at the polarizer P1 141 (which is in this example common to the light source channel and the first detector channel 142).
  • the orthogonal waves of the second detector channel 143 interfere at a second polarizer P2 145.
  • the polarization direction of P2 145 is at 90° to that of P1 141.
  • Two of the single-mode fibers 101 (SMF 2 and SMF 3) guide the light to the photo-detectors 12-1 and 12-2.
  • depolarizers such as the Lyot-type fiber above in the two detector channels after the polarizers 141, 145 in order to avoid polarization dependent losses in the path to the photo-detectors. Such loss could give rise to higher sensitivity to mechanical perturbations of the fibers.
  • depolarizers such as the Lyot-type fiber above in the two detector channels after the polarizers 141, 145 in order to avoid polarization dependent losses in the path to the photo-detectors.
  • SMF 2 and SMF 3 two multimode fibers may be used. Due the larger core size (for example 62.5 ⁇ m instead of the 9- ⁇ m-core of a SMF) the coupling losses from the integrated optical polarization splitter module 14 waveguides into the fibers 101 are reduced.
  • the polarizers 141, 145 can be thin glass platelets containing oriented metal (e.g. silver particles) to polarize the light.
  • a typical thickness of the platelets is for example 30 ⁇ m.
  • a spacer glass platelet 146 with the same thickness as the retarder platelet QWR 144 is used in order to facilitate the assembly of the polarizers P1 141 and P2 145 within the integrated optical module 14.
  • the spacer 146 may consist of glass or can be another QWR platelet with one of its principal axes aligned to the polarization direction of polarizer P1 141, so that it remains inactive and does not affect the polarization of the transmitted light.
  • the spacer 146 can also be a second polarizer platelet with the same orientation as polarizer P1 141, which further enhances the degree of polarization.
  • the QWR 144 is of low order and thus of small thickness (typically a few tens of micrometers). This again helps to limit optical coupling losses.
  • a common polarizer P1 141 for the source light and the first detector channel 142 also facilitates the assembly of the integrated optical module 14, as the waveguides are typically separated at its source side face by only a few 100 ⁇ m.
  • the orientation of P1 141 at 45° to the normal of splitter/combiner SC 140 and hence the PM fiber 132 axis orientation parallel to the splitter/combiner normal is preferred (over an alignment of, e.g., P1 at 0° or 90° and thus a fiber axes at 45° to the SC 140 normal) since potential fiber stress resulting from attaching the PM fiber to the SC tends to be along axes parallel or orthogonal to the SC plane. Disturbing polarization cross-coupling is then minimized.
  • the integrated optical module 14 is preferably part of the sensor head assembly 13. It can be temperature-stabilized as described further below.
  • An important advantage of the configuration of FIG. 1 is that the opto-electronics module 11 can be connected with the sensor head 13 by standard single-mode fibers (or multimode fibers) and standard fiber connectors 102. The more challenging use of a polarization-maintaining fiber for this connection can thus be avoided.
  • the decreasing polarization extinction ratio (PER) of a polarization-maintaining link at increasing cable lengths limits the possible cable length.
  • PM fiber connectors tend to reduce the PER (polarization extinction ratio) in a temperature dependent way and thus can reduce the stability of the sensor scale factor.
  • the cost of PM fiber and PM fiber connectors is substantially higher that the cost of standard singlemode or multimode fibers and corresponding connectors.
  • sensor configurations of the type described above in FIG. 1 or further below are best operated using a signal processing method which is adapted to compensate for asymmetries between the two or more detection channels, as will be described in the following. It should be understood that this signal processing method can be applied to various optical sensors with two or more detection channels and with a static bias optical phase shift as introduced by a passive optical element.
  • N, V, I are the number of windings of the fiber coil, the Verdet constant of fiber ( ⁇ 1 ⁇ rad/A at 1310 nm), and the current, respectively.
  • S 0 is proportional to the light source power.
  • the optical power loss in the two detection channels may differ, e.g. as a result of different coupling loss from the splitter to the fibers SMF2 and SMF3 or due to different loss at fiber connectors.
  • the interference fringe visibility of the two channels may differ as result of tolerances in the relative alignment of the polarizers P1 and P2.
  • the phase difference between the interfering light waves may not be exactly 90 ° due to the temperature dependence of the quarter wave retarder in the two detector channels. Residual birefringence, e.g. due to temperature-dependent stress in the two channels, may introduce further phase offsets.
  • K 1 and K 2 indicate the fringe contrast in the channel 1 and 2, respectively (K 1 and K 2 are equal to unity under ideal conditions and smaller than unity otherwise).
  • ⁇ (T) describes the deviation of the retardation of the QWR 144 from 90° and its variation with temperature.
  • ⁇ (T) and ⁇ (T) describe phase offsets due to other birefringence in the polarization splitter module 14 in channel 1 and channel 2, respectively.
  • the current to be measured is an alternating current (AC) or a transient current (e.g. a current pulse).
  • AC alternating current
  • transient current e.g. a current pulse
  • the measurement of current pulses is of interest for example in the monitoring of correct current commutation in DC breakers (see ref. [5] for details), in the detection of lightning, plasma physics and other.
  • examples of three different methods which can be referred to as AC, transient and DC methods, respectively, are described to account for differential channel loss.
  • the compensation method uses a processed part or representation of the AC signal content. This method is generally preferred for applications with periodic AC currents such as 50 Hz or 60 Hz line currents. Use is made of the knowledge that the amplitudes of the AC contents must be the same in the two channels (apart from being anti-phase) to normalize the two channels to equal signal levels.
  • the largest AC amplitudes S 01,ac and S 02,ac in a certain frequency range are determined by means of fast Fourier transformations (FFT), see FIG. 2A .
  • FFT fast Fourier transformations
  • the FFT will determine the amplitude of the 50 Hz current.
  • LPF low pass filters
  • a measure for the amplitudes S 01,ac and S 02,ac may be determined by a series of a high pass filter (HPF1), a rectifier (R), and a low pass filter as shown in FIG. 2B .
  • HPF1 high pass filter
  • R rectifier
  • R low pass filter
  • the high pass filters cut off the DC signal contents.
  • the rectifiers outputs are then proportional to the amplitudes S 01,ac and S 02,ac and the low pass filters again serve to time average the signals.
  • signal S 2 may be left unchanged and signal S 1 is multiplied by the amplitude ratio S 02 , ac / S 01,ac , i.e. by the inverse of A) .
  • the signals S 1 and S 2 have the same amplitude, i.e. are normalized to equal power loss.
  • FIG. 2C shows a modification of the method illustrated by FIG. 2A .
  • signal S 1 is multiplied in a multiplier (X) with amplitude S 02
  • ac and signal S 2 is multiplied in a second multiplier (X) with amplitude S 01,ac .
  • signals S 1 and S 2 are again normalized to equal power loss.
  • An advantage over the methods as illustrated by FIGs. 2A and 2B can be seen in that the normalization does not require any signal division and hence requires less signal processing power.
  • phase offsets ⁇ and ⁇ are assumed to be negligible. Also it should be noted that the particular value of ⁇ (deviation of the QWR retarder from 90°) does not affect the recovered phase shift as long as ⁇ «1.
  • the measurement signal S after the second divider (/) can advantageously be high pass filtered, as indicated by the filters HPF, HPF2 shown dashed in FIGs. 2A - 2C .
  • the cutoff frequency is chosen sufficiently small so that the system is able to detect all desired AC and transient content.
  • the cutoff frequency may be chosen for example in the range between 0.001 Hz and 10 Hz.
  • With a low cut-off frequency e.g. 0.001 Hz
  • the HPF1 cutoff frequency in FIG. 2B may be the same as the HPF2 cutoff frequency or the two frequencies may differ, e.g. the cutoff frequency of HPF1 may be closer to the rated AC frequency than the cutoff frequency of HPF2.
  • the LPF cutoff frequency (or equivalently the time span over which the ratio A is averaged) is best chosen such that on one hand random fluctuations of A due to signal noise are kept small, and that on the other hand the response to optical power variations, e.g. as a result of temperature variations or fiber movement, is sufficiently fast.
  • An appropriate averaging time may be in the range between 1 s and 100 s (seconds). A preferred value is in the range of 1 s to 20 s.
  • band pass filters centered around the rated current frequency can be used.
  • the two input signals of the adder (+) can be low pass filtered in order to lower the noise in the adder's output.
  • the compensation methods as described above can also be applied to the case of transient currents, such as current pulses.
  • the ratio A is then determined by dividing in the divider (/) the instantaneous (digital or analog) outputs of the two high pass filters HPF1 ( FIG. 2B ).
  • the HPF1 cutoff frequency is adapted to the expected current rise and fall times.
  • the normalization is only active as long as the current is above a set threshold in order to avoid erroneous normalization due to signal noise. Below the threshold, a preset default value of A or the last valid value of A may be used.
  • FIG. 3 there is described a DC based method for compensating the asymmetry between different detector channels.
  • the DC signal contents S 1,dc and S 2,dc are used to normalize the signals with regard to differential optical loss.
  • This method is preferred in case of transient currents such as current pulses which may occur at random times, i.e. the method may be used when no AC signal part is available for normalization.
  • a condition is that there is also no continuous dc current of significant magnitude flowing as dc would introduce anti-phase offsets in the two signals, which would distort the normalization.
  • Non-negligible phase offsets ⁇ (T), ⁇ (T), and ⁇ (T) will affect the accuracy of the procedure. If the sum of the offsets is determined by an independent measurement, the offsets can be taken into account by multiplying one of the Signals S 1 , S 2 with an appropriate correction factor.
  • Such a temperature compensation can be achieved through a temperature controlled environment and/or through an extraction of the temperature from at least one detector signal, examples of which are described further below.
  • a method as described in reference [3] for fiber coils free of linear birefringence or of low birefringence can be used.
  • the temperature dependence of a retarder such as the fiber retarder 133 at the beginning of the sensing fiber 131 as shown in the example of FIG. 1 is employed to compensate the temperature dependence of Faraday effect.
  • the angle ⁇ is an appropriately chosen deviation of the retarder 133 from perfect ⁇ /2-retardation at a reference temperature, e.g. room temperature.
  • the method can be extended in the case of AC measurements to include a further method for determining the temperature of the passive elements, such as the integrated optical module 14 above. It should be noted that this method can be applied to many different optical sensors for an AC or transient measurand and can thus be considered an independent aspect of the present invention. Further embodiments of optical sensors that can use this invention are described further below.
  • the temperature of the QWR 144 can be extracted from the low pass filtered sensor output, preferably after normalization of the detector signals by means of the AC signal contents as shown in FIG. 4 , which also includes the elements already described when referring to FIG. 2A above.
  • ⁇ (T) and ⁇ (T) are assumed as sufficiently small and the fringe contrast K is assumed as being equal for the two channels.
  • phase shifts from potential dc currents are small compared to ⁇ over the LPF2 signal averaging time.
  • S dc can serve also as a measure for the sensor head 13 temperature and can be used to compensate any (remaining) variation of the sensor head scale factor with temperature.
  • the quarter wave retarder QWR 144 of the integrated optical module 14 is commonly a quartz-platelet.
  • the retardation varies by about 0.5° over a temperature range of 100°C at a wavelength of 1310 nm.
  • the variation should be significantly larger than any potential contribution from the terms ⁇ (T) and ⁇ (T).
  • the temperature can still be determined from the output of LPF2, as long as the signal varies monotonically with changing temperature and is appropriately calibrated in terms of temperature.
  • a linearization of the sensor signal as represented by eq. (13) can be included in the signal processing. Furthermore, deviations of the bias phase shift from 90°, particularly the influence of temperature on ⁇ , can be taken into account for this linearization as a zero-point correction.
  • ⁇ (T) as retrieved from S dc can be included in the determination of the AC phase shift ⁇ ac from the signal S (according to eq. [13]).
  • the phase terms can be kept stable. Their sum as well as the contrast K can be determined by calibration.
  • Residual temperature-dependent stress e.g. from adhesives, that can cause unwanted birefringence in the polarization splitter module 14, in particular at the retarder, can remain as a limitation of the sensor accuracy.
  • stress may affect the contrast terms K 1 and K 2 through polarization cross-coupling between the interfering light waves as well as the phase terms ⁇ (T) and ⁇ (T) as referred to in the equations (7),(8) above.
  • FIG. 5 shows an arrangement of a sensor according to FIG. 1 , wherein influences of the integrated optical polarization splitter module 14 on the scale factor stability over temperature are essentially eliminated.
  • the sensor head 13 is located at the high electric potential side of a free-standing electric insulator 17, i.e. the insulator 17 and sensor head 13 represent a free-standing device equivalent to a conventional instrument current transformer.
  • the integrated optical polarization splitter module 14 is positioned at the ground potential side of the insulator 17 and is connected with the fiber coil 131 by a polarization-maintaining fiber link 132.
  • the link 132 runs through the hollow-core of the insulator 17.
  • the arrangement maintains the advantage of single-mode (or multi-mode) fiber links between the location of the insulator 17 and hence the integrated optical polarization splitter module 14 and the opto-electronic module 11 of the sensor 131.
  • the PM fiber link 132 only extends over the distance from ground to high voltage potential.
  • the temperature of the integrated optical module 14 is kept at a temperature that corresponds to or is near the highest temperature of operation.
  • the maximum temperature of operation is 65°C
  • the integrated optical module 14 can be kept in the range between 65°C and 45°C at ambient temperatures between 65°C and -40°C.
  • the temperature control requires only provisions for heating but none for cooling.
  • the integrated optical module 14 may be placed in a thermally insulated package or housing 18 as shown.
  • the temperature is stabilized by means of a self-regulated heating foil resistor 181.
  • the resistor material has a strong positive thermal coefficient and acts as a "thermal diode".
  • the heat power at a given voltage for example 24 V
  • the temperature may be stabilized by means of one or several heating resistors (not shown) with the current controlled by electronics.
  • the temperature of the integrated optical module 14 can be controlled by means of a thermo-electric cooler/heater that is able to maintain an arbitrary constant temperature, e.g. 25°C.
  • the insulator 17 is a hollow-core insulator consisting of hollow fiber reinforced epoxy tube. Silicone sheds on the outer surface provide sufficient creepage distance between high voltage and ground to prevent flash-over, e.g. in case of pollution by rain water or dirt.
  • the PM fiber 132 is for example protected by a fiber cable comprising an inner gel-filled tube that contains the fiber. The gel filling prevents excessive fiber stress and thus unwanted polarization cross-coupling between the two orthogonal polarization modes, e.g. due to differential thermal expansion.
  • the insulator bore is filled with a soft insulating material 172, e.g. silicone, which provides sufficient dielectric strength.
  • the silicone contains a filler material which has sufficient compressibility and accommodates any thermal expansion of the silicone.
  • the filler can for example consist of micron sized beads made of a soft material or of tiny fluid bubbles or gas bubbles.
  • the bubbles may contain sulfur hexafluride (SF 6 ) gas or alternative dielectric insulation fluid mixtures or gas mixtures comprising an organofluorine compound, such organofluorine compound being selected from the group consisting of: a fluoroether, an oxirane, a fluoroamine, a fluoroketone, a fluoroolefin, and mixtures and/or decomposition products thereof.
  • the dielectric insulation medium can further comprise a background gas different from the organofluorine compound and can in embodiments be selected from the group consisting of: air, N 2 , O 2 , CO 2 , a noble gas, H 2 ; NO 2 , NO, N 2 O; fluorocarbons and in particular perfluorocarbons, such as CF 4 ; CF 3 I, SF 6 ; and mixtures thereof.
  • a background gas different from the organofluorine compound can in embodiments be selected from the group consisting of: air, N 2 , O 2 , CO 2 , a noble gas, H 2 ; NO 2 , NO, N 2 O; fluorocarbons and in particular perfluorocarbons, such as CF 4 ; CF 3 I, SF 6 ; and mixtures thereof.
  • the insulator 17 can be filled with polyurethane foam and/or contain an insulating gas such as nitrogen (N 2 ) or sulfur hexafluride (SF 6 ) or alternative dielectric insulation gas mixtures comprising an organofluorine compound, such organofluorine compound being selected from the group consisting of: a fluoroether, an oxirane, a fluoroamine, a fluoroketone, a fluoroolefin and mixtures and/or decomposition products thereof.
  • an insulating gas such as nitrogen (N 2 ) or sulfur hexafluride (SF 6 ) or alternative dielectric insulation gas mixtures comprising an organofluorine compound, such organofluorine compound being selected from the group consisting of: a fluoroether, an oxirane, a fluoroamine, a fluoroketone, a fluoroolefin and mixtures and/or decomposition products thereof.
  • the dielectric insulation gas can further comprise a background gas different from the organofluorine compound and can in embodiments be selected from the group consisting of: air, N 2 , O 2 , CO 2 , a noble gas, H 2 ; NO 2 , NO, N 2 O; fluorocarbons and in particular perfluorocarbons, such as CF 4 ; CF 3 I, SF 6 ; and mixtures thereof.
  • the gas can be at atmospheric pressure or at elevated pressure to enhance its dielectric strength.
  • the insulator can include of a solid inner rod of fiber reinforced epoxy with the PM fiber running inside a capillary along a helical path along the outer surface of insulator rod as disclosed for example in reference [8].
  • the temperature stabilized housing 18 containing the integrated optical module 14 can be mounted for example in an external housing 182 that is attached to the insulator flange 171 as shown.
  • the external housing 182 protects the fiber leads of the integrated optical module 14 and is equipped with fiber connectors 102 for the fiber cable 101 between the sensor opto-electronic module 11 and the HV insulator 17. Furthermore, the external housing 182 acts as a sun-shield and mechanical protection of the integrated optical module 14.
  • a connector shield 103 protects the connectors 102 and has provisions for strain relief 104 of the fiber cable 101.
  • the fiber coil housing or sensor head 13 is horizontally mounted between the terminal plates 151 for the power line cables 15.
  • An insulating layer 152 prevents the current to pass outside the fiber coil 131.
  • the fiber coil can be mounted in a vertical position on top of the insulator with the current passing the coil in horizontal direction.
  • the housing can also be attached to the terminal of a high voltage circuit breaker.
  • the polarization maintaining fiber link to ground may be designed as a flexible high voltage fiber cable equipped with sheds to enhance the creepage distance.
  • the temperature controlled splitter can then be mounted in the breaker drive cubicle or in a separate enclosure nearby.
  • the fiber coil housing or sensor head 13 can also be mounted inside the circuit breaker on top of the circuit breaker support insulator, as described for example in reference [9].
  • the PM fiber link runs to ground through the gas volume (with any dielectric insulation medium as disclosed above) of the support insulator and leaves the support insulator through a gas-tight fiber feed-through.
  • the temperature controlled module 14 can again be mounted in the breaker drive cubicle or in a separate enclosure nearby. Further alternatives to mount the fiber coil housing or sensor head 13 in high voltage circuit breakers are disclosed in reference [10].
  • current may be measured on ground potential with sensor arrangements equivalent to the ones as disclosed earlier for gas-insulated high voltage switchgear (GIS) (see for example reference [11] for further details), generator circuit breakers (see for example reference [12] for further details), or bushing of a HVDC converter station (see for example reference [13] for further details).
  • GIS gas-insulated high voltage switchgear
  • generator circuit breakers see for example reference [12] for further details
  • bushing of a HVDC converter station see for example reference [13] for further details.
  • FIGs. 6A and 6B show the scale factor variation versus the sensor head temperature for the cases that the both the fiber coil 131 and module 14 are exposed to the same temperature ( FIG. 6A ) and that the module 14 is temperature-stabilized according to the methods proposed herein ( FIG. 6B ).
  • the fiber coil 131 is temperature compensated by means of the fiber retarder 133 (as described in detail when referring to eq. (12)).
  • the remaining scale factor variation between -40 and 85°C is reduced from about 0.5% to ⁇ 0.1% and thus meets common requirements for metering ( ⁇ 0.2%).
  • any of the above-described methods for normalization and temperature stabilization can be applied to different types of optical sensors, which can be similar or different from the sensor described in connection with FIGs. 1 and 5 .
  • Examples of other possible optical sensor configuration having a passive element to introduce a static bias optical phase shift between two detector channels are described in the following. It should be noted that the examples presented are only representative and the application of the above methods is not limited to them.
  • the element to introduce a static bias optical phase shift is a Faraday rotator, in particular, a Faraday rotation mirror 144' located at the end of the sensing fiber 131.
  • the conversion of the phase shift to optical power changes of opposite phase in two output channels 142, 143 is achieved by a polarizing beam splitter 16.
  • Stress and misalignment at the polarizing beam splitter 16 can yield quantities K 1 (T), K 2 (T) in eqs. (7) and (8) deviating from 1 and quantities ⁇ (T) and ⁇ (T) in eqs. (7) and (8) deviating from zero.
  • FIG. 7A There are two options as to how the sensor shown in FIG. 7A can be operated.
  • two orthogonal linear polarization states are sent through the PM fiber 132 to the sensing fiber 131 and converted by means of a fiber retarder 133 to left and right circular light waves at the entrance to the fiber sensing fiber 131.
  • only one linear polarization state, generated at the polarization splitter is sent through the PM fiber 132 and further (without conversion into circular polarization) into the sensing fiber 131.
  • the current signal can be retrieved from the detected light powers in the same way as in the previous sensor embodiment of FIG. 1 , with the two detector channels 142, 143 being formed by the two signals as generated when the light from the sensing fiber 131 passes through beam splitter 16.
  • Temperature stabilization of elements 16 and 144' as described in this invention improves the temperature stability of the sensor signal.
  • a third example of a fiber-optic current sensor with passive optical elements and also having a sensor characteristic according to eq. (7) is described in the following.
  • This transmission-type sensor configuration is schematically depicted in FIG. 7B .
  • the linear polarizer 141 generates linearly polarized light that is injected into the sensing fiber 131 (in this example a principal axis of the PM fiber pigtail 132-1 is parallel to the polarizer orientation).
  • the magneto-optic phase shift in the sensing fiber 131 becomes manifest as a rotation of light polarization.
  • the phase biasing through a passive element 144 is here achieved by the orientation of the polarizing beam splitter 16 at ⁇ 45° to the axis of the first linear polarizer 141 (the axes of the PM fiber pigtail 132-2 of the polarizing beam splitter 16 are also oriented at ⁇ 45° with respect to the axis of the first polarizer 141) which also splits in the incoming light into two orthogonal linear polarization states.
  • the detected light power in the two channels 142, 143 is again described by eqs. (7) and (8) with ⁇ being smaller by a factor of 2 compared to the previous examples since the sensing coil 131 is only passed once.
  • (7) and (8) can deviate from 1 and quantities ⁇ (T), ⁇ (T) and ⁇ (T) can deviate from 0 due to imperfections of the linear polarizer 141, the polarizing beam, or their relative angular orientation. Temperature stabilization of element 16 as described in this invention improves the temperature stability of the sensor signal.
  • FIG. 8 depicts an optical voltage sensor to which the above methods can be applied.
  • the detection system comprises to a large extent the same components as the first example of a fiber-optic current sensor employing an integrated optical polarization splitter module 14, as already described when referring to FIG. 1 above.
  • the sensing coil 131 fiber retarder, sensing fiber, fiber tip reflector
  • an electro-optic element 134 terminated at the far end by a reflector 135, and at the near end by a 45°-Faraday rotator 133'.
  • the reflector 135 can be implemented using, e.g., a mirror, a reflective coating, a reflective prism, or a corner cube reflector.
  • the electro-optic element 134 can be a rod shaped electro-optic Bi3Ge4012 (BGO) crystal 134.
  • BGO electro-optic Bi3Ge4012
  • an electro-optic fiber 134 may be used, such as a crystalline fiber 134 as described in reference [19] or an electrically poled fiber 134 as described in reference [20].
  • the Faraday polarization rotator 133' at the electro-optic element's near end rotates the two orthogonal light waves emerging from PM fiber 132 by 45° before they enter the electro-optic crystal.
  • the polarization directions after the rotator 133' coincide with the electro-optic axes of the crystal 134.
  • the light is reflected at the far end of the crystal 134 by means of reflector 135.
  • the two orthogonal light waves experience a differential electro-optic phase shift in the crystal 134 that is proportional to the applied voltage.
  • the Faraday rotator 133' rotates the returning light waves by another 45° so that the total roundtrip polarization rotation corresponds to 90°. (The polarization rotation is needed so that the roundtrip group delay of the orthogonal polarization states in PM fiber 132 is zero and the two waves are again coherent when they interfere at the polarizers 141, 145)
  • the electro-optic phase shift is extracted analogously to the magneto-optic phase shift of the fiber-optic current sensor of FIG. 1 .
  • the accuracy enhancing aspects of the invention as already discussed in further details and examples above can be applied as in the case of fiber-optic current sensors.
  • the temperature stabilization of the integrated optical polarization splitter module 14 increases the signal stability analogously to the case of the fiber-optic current sensors.
  • the electro-optic sensing element 134 as illustrated in Fig. 8
  • other designs of the voltage sensing element as described in Ref. 16 may be used, as well.
  • the various aspects of the present invention can analogously be applied to an optical voltage sensor based on the piezo-electric effect in materials such as quartz.
  • the quartz element(s) strain(s) an attached PM sensing fiber in the presence of an applied voltage and as a result introduce(s) again a voltage-dependent phase shift between the orthogonal polarization states of the sensing fiber (see ref. [16, 7] for further details).
  • the PM sensing fiber may also in similar manner act as a sensor for strains or forces of other origin.

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KR102159420B1 (ko) 2020-09-24
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